1. Field of Invention
The present invention relates to the smelting reduction of iron ore in a blast furnace.
2. Discussion of Prior Art
Conventional steel production methods make extensive use of fossil fuels. Use of these fuels leads to many undesired side products such as carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, and others. Many of these side products are released into the atmosphere, for example, through smoke stacks. The use of fossil fuels has a detrimental impact on the environment: acid rain, airborne carcinogens, global warming, and smog are all partially attributed to fossil fuel use. To account for the externalities of fossil fuels, various carbon taxes have been proposed. If such taxes become law, then there will be a strong profit motive to move away from fossil fuels.
The most energy intensive step in steel making is the initial reduction of the iron ore to the metallic state. Most commonly, this reduction step is accomplished through the use of a blast furnace. These furnaces consume vast quantities of coal (i.e. fed as coke), both to heat the reduction reaction and to supply the reductant gas. Alternatives to the blast furnace process have used natural gas for the reduction process. These alternate processes have led to greater energy efficiency and lower carbon emissions. However, the products of direct-reduction processes are not generally usable in steel-conversion processes which expect blast furnace metal, owing to the low carbon content of the reduction product. Additionally, carbon emissions can not be eliminated through the use of alternative fossil fuels such as natural gas.
Another possibility which is a well-known process is the injection of hot reducing gas at the level of the main tuyeres of the blast furnace in order to reduce coke consumption. This reducing gas contains primarily CO, H.sub.2, and possibly N.sub.2, as well as small amounts of CO.sub.2 and H.sub.2 O. In U.S. Pat. No. 4,421,553 to Ponghis et al. (1983), this process is taught in detail. Ponghis et al. teach not only the reduction process, but methods for controlling several blast furnace parameters, including pig iron temperature, Si content, coke rate, hot metal production rate, and top gas temperature. While this patent discloses coke rate reductions of up to 85%, carbon dioxide emissions remain high as the coke used is replaced by other carbonaceous fuels.
In a paper by J. Gretz et al., published in the International Journal of Hydrogen Energy, volume 16, number 10, pages 691-693 (1991) hydrogen produced via electrolysis from hydroelectric power was proposed as a steel plant fuel. Although this paper presents a viable alternative to fossil fuels, it does not present any method for the actual use of hydrogen in this fashion.